The clinical diagnostic field has seen a broad expansion in recent years, both as to the variety of materials of interest that may be readily and accurately determined, as well as the methods for the determination. Over the last decade, testing for drugs of abuse has become commonplace. This testing is not only for the monitoring of criminal offenders and drug addicts, but employers also use it for the screening of workers. In recent years, immunoassay based on the reaction of an antibody with an antigen has been extensively investigated for this purpose. Some categories of immunoassays include a radioimmunoassay, using a radioactive isotope, an enzyme-based immunoassay (EIA) using an enzyme, and luminescence assays, using fluorescent labels, e.g., fluorescence polarization, and chemiluminescent labels.
The clinically important and potent analgesic buprenorphine, a semi-synthetic narcotic derived from thebaine, has attracted attention as a potential pharmacotherapy for opiate abuse and dependence. At times of shortage of heroin, buprenorphine, BUP hereinafter may be used as the substitute drug of choice. BUP's unique effects and pharmacology make it an attractive and clinically helpful treatment option. The unique pharmacology of buprenorphine at the mu-opioid receptor (i.e. high affinity, low intrinsic activity and slow dissociation) results in buprenorphine having: (1) a good safety profile, (2) low physical dependence, and (3) flexibility in dose scheduling. Further, BUP produces less euphoria than morphine and heroin. When compared with other opiates, it also causes a significantly lower degree of sedation and respiratory depression, the slowing down of breathing that makes heroin overdoses so dangerous.
On the other hand, BUP has been chosen by the National Institute on Drug Abuse as one of the medications for the treatment of opiate dependence. Norbuprenorphine, norBUP hereinafter, is a major dealkylated metabolite of BUP and displays similar but distinct properties. These limited side effects can be attributed to the fact that BUP and norBUP are partial mu agonists. Agonists are chemicals that bind to and stimulate opiate receptors. Antagonists block the effects of opiates by binding to receptors without stimulating them. By stimulating mu opiate receptors in the brain, mu agonists produce the effects associated with morphine: analgesia, euphoria, sedation, and respiratory depression. Because BUP is a partial mu agonist, it also readily binds to mu opiate receptors. However, BUP activates these receptors to a lesser degree than full mu receptor agonists such as morphine and heroin. Both norBUP and BUP are potent partial agonists, with norBUP having moderate efficacy and BUP having low efficacy. BUP also is released slowly from the mu receptor, producing a long-lasting effect. Therefore, it may be possible to give BUP to patients every other day, rather than in the daily doses that methadone patients must receive. Some studies also suggest that withdrawal effects are less severe with BUP than with methadone. From both a drug of abuse and therapeutic drug monitoring view point, it is important to have available reliable immunoassays for BUP and its metabolite, norBUP. HPLC-MS methods for the detection of buprenorphine and norbuprenorphine are disclosed in the literature [Kronstrand, R.; Selden G.; Josefsson, M. “Analysis of buprenorphine, norbuprenorphine, and their glucuronides in urine by liquid chromatography-mass spectrometry”, J. Anal. Toxicol., 2003, 27, (7), p464–70.], and two immunoassays have also been disclosed [Cirimele, V.; Kintz, P.; Lohner, S; Ludes, B. “Enzyme immunoassay validation for the detection of buprenorphine in urine”, J. Anal. Toxicol., 2003, 27, (2), p103–5, and Tiong, G. K. L.; Olley, J. E. “Enzyme immunoassay of buprenorphine”, Naunyn-Schmiedeberg's Arch Pharmacol 1988, 338, 202] However, these assays are not suitable for use in automated clinical analyzers. There is, therefore, a need for assays for the detection of the aforementioned drugs and, in some instances, their major metabolites. The assays should be able to detect these drugs in order to monitor and treat addicted patients.